The building blocks of a spherical basis are given by a set of base
vectors \(\mathbf {e}_m\)  (\(m=0,\pm \) )
[1]}, [2]}
\(\mathbf {e}_0 = \hat{\mathbf {e}}_3, \quad \mathbf {e}_\pm = \mp \frac{\hat{\mathbf {e}}_1 \pm i\hat{\mathbf {e}}_2}{\sqrt{2}},\) 
